The present invention is directed to antennas. It is particularly advantageous in direction-finding antennas, although it can be applied to other types of antennas as well.
It is often required of direction-finder antenna systems that they be capable of covering the entire 360.degree. azimuthal range at and a little above the elevation of the horizon. In the past, most devices for achieving this purpose have been limited to a very narrow bandwidth. Consequently, when devices of this type were employed, a large number of them were needed if the frequency band to be monitored was wide.
An antenna whose characteristics are relatively frequency independent throughout a broad bandwidth is the log-periodic antenna. In such an antenna, the individual radiating elements are disposed along and perpendicular to an axis. The dimensions of the individual elements are proportional to the distance of the element from a reference point, or vertex, on the axis, and the distances between adjacent elements along the axis are also proportional to the distance from the vertex so that the ratio of the dimensions of one element to those of the previous adjacent element in the array is the same as the ratio for any two other adjacent elements.
Although this log-periodic structure results in a relatively frequency-independent response, radially orienting a number of such structures as subarrays of a composite array to achieve a 360.degree. range has not in the past proved satisfactory. The interaction between the individual log-periodic subarrays has resulted in direction-finding errors. Thus, it was previously necessary to employ either a narrow-band device to achieve the 360.degree. range, to use extensive azimuth, elevation, and polarization antenna-response calibrations, or to limit the log-periodic structure to a single log-periodic array and thereby achieve the frequency-independent response without the 360.degree. coverage in a single device.